1. Field of the Invention
This invention relates generally to implantable medical devices, and, more particularly, to methods, apparatus, and systems for performing an override of a normal operation of an implantable medical device.
2. Description of the Related Art
There have been many improvements over the last several decades in medical treatments for disorders of the nervous system, such as epilepsy and other motor disorders, and abnormal neural discharge disorders. One of the more recently available treatments involves the application of an electrical signal to reduce various symptoms or effects caused by such neural disorders. For example, electrical signals have been successfully applied at strategic locations in the human body to provide various benefits, including reducing occurrences of seizures and/or improving or ameliorating other conditions. A particular example of such a treatment regimen involves applying an electrical signal to the vagus nerve of the human body to reduce or eliminate epileptic seizures, as described in U.S. Pat. Nos. 4,702,254, 4,867,164, and 5,025,807 to Dr. Jacob Zabara, which are hereby incorporated in their entirety herein by reference in this specification.
More generally, the endogenous electrical activity (i.e., activity attributable to the natural functioning of the patient's own body) of a neural structure of a patient may be modulated in a variety of ways. In particular, the electrical activity may be modulated by exogenously applied (i.e., from a source other than the patient's own body) electrical, chemical, or mechanical signals applied to the neural structure. The modulation (hereinafter referred to generally as “neurostimulation” or “neuromodulation”) may involve the induction by the generation of afferent action potentials, efferent action potentials, or both, in the neural structure, and may also involve blocking or interrupting the transmission of endogenous electrical activity traveling along the nerve. Electrical neurostimulation or modulation of a neural structure refers to the application of an exogenous electrical signal (as opposed to a chemical or mechanical signal), to the neural structure. Electrical neurostimulation may be provided by implanting an electrical device underneath the skin of a patient and delivering an electrical signal to a nerve such as a cranial nerve. The electrical neurostimulation may involve performing a detection, with the electrical signal being delivered in response to a detected body parameter. This type of stimulation is generally referred to as “active,” “feedback,” or “triggered” stimulation. Alternatively, the system may operate without a detection system once the patient has been diagnosed with epilepsy (or another medical condition), and may periodically apply a series of electrical pulses to the nerve (e.g., a cranial nerve such as a vagus nerve) intermittently throughout the day, or over another predetermined time interval. This type of stimulation is generally referred to as “passive,” “non-feedback,” or “prophylactic,” stimulation. The stimulation may be applied by an implantable medical device that is implanted within the patient's body, or by a device that is external to the patient's body, with a radio frequency (RF) coupling to an implanted electrode.
Generally, implantable medical devices (IMD) are capable of receiving a signal that may affect the operation of the IMD, from sources external to the IMD, such as a patient-initiated signal or a signal in the patient's environment. For example, a magnetic sensor may be provided in the IMD to detect a significant magnetic field, and in response, activate a predetermined function. A magnetic signal input from a patient may include an inhibitory input or an excitatory input. The inhibitory input may relate to inhibiting a function normally performed by the IMD. For example, application of a particular magnetic field to the IMD may cause delivery of the electrical signal from the IMD to the nerve to be inhibited for a certain time period. Application of a different magnetic field signal to the IMD may prompt the IMD to perform additional functions. For example, additional stimulation therapy delivery may be performed by the IMD based upon a particular magnetic signal input. The magnetic signal input may be generated by a patient by placing a magnet proximate the skin area under which the implantable medical device resides in the human body. Both types of magnetic field signals are typically referred to as “magnet modes” or as “magnet mode” operation.
One problem associated with current magnet mode approaches results from external magnetic fields that are not intended by the patient to function as a magnetic signal input to the IMD. Thus, if a patient encounters an external magnetic field, such as a magnetic resonance imaging (MRI) signal, or other strong magnetic or electromagnetic fields, normal operations performed by the stimulation by the IMD may be affected. This could cause inadvertent inhibition of the delivery by the IMD of the electrical signal to the nerve, or inadvertent alteration of the neurostimulation therapy. A person entering an area of magnetic activity or fluctuations may cause an IMD to experience false inputs. Current IMD configurations generally lack an effective method of overriding such false inputs.
The present invention is directed to overcoming, or at least reducing, the effects of one or more of the problems set forth above.